dts: arm: gigadevice: gd32f403: add timer and pwm nodes

Add timer and pwm nodes. The structure follows a similar approach to the
one used by STM32. Timer nodes store all timer information (e.g.
pre-scaler), whereas the pwm nodes are children of the timer node
indicating a specialization of the multiple timer functionalities.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>